Another way to convert by steps from one image to another is morphing. You can use xmorph to do that.
Fade from teapot.ppm to pyr.ppm generating fade.0001.ppm to fade.0030.ppm using the "spread" transition.
ppmfade -l teapot.ppm
Fade from black to teapot.ppm generating fade.0001.ppm to fade.0030.ppm.
ppmfade -f teapot.ppm -base end
Fade from teapot.ppm to black generating end.0001.ppm to end.0030.ppm.